1. Field of the Invention
The present invention relates to methods of monitoring the cardiac output or pulmonary capillary blood flow of a patient. More specifically, the present invention relates to methods of continuously monitoring the cardiac output or pulmonary capillary blood flow of a patient during each breath or respiratory cycle, flow, and, particularly, to methods of continuously, non-invasively determining cardiac output or pulmonary capillary blood flow. The present invention also relates to methods of monitoring the cardiac output or pulmonary capillary blood flow of a patient during both stable ventilation or breathing and during or following changes in ventilation or breathing.
2. Background of Related Art
Conventionally, cardiac output has been measured both intermittently and continuously. Intermittent techniques of measuring cardiac output include invasive and non-invasive techniques.
Indicator dilution, an exemplary invasive, typically intermittent technique for measuring cardiac output, includes introducing a predetermined amount of an indicator into the bloodstream through the heart of a patient and analyzing blood downstream from the point of introduction to obtain a time vs. dilution curve. Thermodilution, in which room temperature or colder saline solution, which may be referred to as "cold" saline, is employed as the indicator, is a widely employed type of indicator dilution. Typically, the cold saline is introduced into the right heart bloodstream of a patient through a thermodilution catheter, which includes a thermistor at an end thereof. The thermistor is employed to measure the temperature of the blood after it has passed through the right heart, or downstream from the point at which the cold saline is introduced. A thermodilution curve is then generated from the data, from which the cardiac output of the patient may be derived. Thermodilution and other indicator dilution techniques are, however, somewhat undesirable due to the potential for harm to the patient that is associated with inserting and maintaining such catheters in place.
One of the less invasive conventional techniques for measuring the cardiac output of a patient employs the Fick principle: the rate of uptake of a substance by or release of a substance from blood at the lung is equal to the blood flow past the lung and the content difference of the substance at each side of the lung.
The Fick principle may be represented in terms of oxygen (O.sub.2) by the following formula: EQU Q.sub.t =VO.sub.2 /(CaO.sub.2 -CvO.sub.2),
where Q.sub.t is the cardiac output, or blood flow, of the patient, VO.sub.2 is the net volume of oxygen consumed by the patient per unit of time, CaO.sub.2 is the content of O.sub.2 in the arterial, or oxygenated, blood of the patient, and CvO.sub.2 is the content of O.sub.2 in the venous, or de-oxygenated, blood of the patient. The oxygen Fick principle may be employed in calculating the cardiac output of a patient either intermittently or continuously.
An exemplary, so-called "non-invasive", method of determining the cardiac output of a patient by monitoring VO.sub.2 is disclosed in Davies et al., Continuous Fick cardiac output compared to thermodilution cardiac output, Crit. Care Med. 1986; 14:881-885 ("Davies"). The method of Davies includes continually measuring the O.sub.2 fraction of samples of gas inspired and expired by a patient, the oxygen saturation (SvO.sub.2) of the patient's venous blood, and oxygen saturation (SaO.sub.2) of the patient's arterial blood. The O.sub.2 measurements are made by a metabolic gas monitor, and VO.sub.2 calculated from these measurements. SaO.sub.2 is measured by pulse oximetry. SvO.sub.2 may be directly measured by a pulmonary artery ("PA") catheter equipped to measure oxygen saturation. Each of these values is then incorporated into the oxygen Fick equation to determine the cardiac output of the patient. Although the method of Davies may be employed to intermittently or continuously determine the cardiac output of a patient, it is somewhat undesirable from the standpoint that accurate VO.sub.2 measurements are typically difficult to obtain, especially when the patient requires an elevated fraction of inspired oxygen (FiO.sub.2). Moreover, since the method disclosed in Davies requires continual measurement of SvO.sub.2 with a pulmonary artery catheter, it is, in actuality, an invasive technique.
Due in part to the ease with which the carbon dioxide elimination (VCO.sub.2) of a patient may be accurately measured, VCO.sub.2 measurements are widely employed in methods of non-invasively determining the cardiac output of a patient. Since the respiratory quotient (RQ) is the ratio of carbon dioxide elimination to the amount of oxygen inhaled, VCO.sub.2 may be substituted for VO.sub.2 according to the following exemplary equation: EQU VO.sub.2 =VCO.sub.2 /RQ.
An exemplary method of continuously measuring the cardiac output of a patient in terms of CO.sub.2 is disclosed in U.S. Pat. No. 4,949,724 ("the '724 patent"), which issued to Mahutte et al. on Aug. 21, 1990. The method of the '724 patent employs the following form of the Fick equation to determine the cardiac output of a patient: EQU Q.sub.t =VCO.sub.2 /(Hgb.multidot.RQ.multidot.(SaO.sub.2 -SvO.sub.2)),
where VCO.sub.2 /(Hgb.multidot.RQ.multidot.(SaO.sub.2 -SvO.sub.2)) has been substituted for VO.sub.2 /(CaO.sub.2 -CvO.sub.2) and Hgb is the concentration of hemoglobin in the blood (typically about 13.4 g/dl). A constant, k, may be employed to replace either Hgb or Hgb.multidot.RQ.
According to the method of the '724 patent, an initial cardiac output measurement is made by thermodilution techniques. Thereafter, k is calculated. Subsequently, a CO.sub.2 flowmeter and monitor are employed to measure VCO.sub.2, SvO.sub.2 is measured with a catheter and oximetry processor, and SaO.sub.2 is measured by a pulse oximeter. The cardiac output of the patient may be continuously calculated as described above. The method of continuously measuring cardiac output of the '724 patent is, however, somewhat undesirable as the use of a catheter to initially determine cardiac output and to continuously measure SvO.sub.2 is invasive and may thus create additional health risks for the patient.
Alternatively, a modification of the Fick principle, which is based on the exchange of carbon dioxide (CO.sub.2) in the lungs of a patient, has been employed to calculate the cardiac output of the patient. The carbon dioxide Fick equation, which represents the Fick principle in terms of CO.sub.2 production and exchange, follows: EQU Q.sub.1 =VCO.sub.2 /(CvCO.sub.2 -CaCO.sub.2),
where CvCO.sub.2 is the content of CO.sub.2 in the venous blood of the patient and CaCO.sub.2 is the content of CO.sub.2 in the arterial blood of the patient. The difference between CvCO.sub.2 and CaCO.sub.2 is typically referred to as the arterial-venous carbon dioxide gradient, "AV CO.sub.2 gradient", or simply "AV gradient".
The carbon dioxide Fick equation has been employed to non-invasively determine the cardiac output of a patient on an intermittent basis. The carbon dioxide elimination of the patient may be non-invasively measured as the difference per breath between the volume of carbon dioxide inhaled during inspiration and the volume of carbon dioxide exhaled during expiration. The volume of carbon dioxide inhaled and exhaled may each be corrected for any deadspace. The partial pressure of end-tidal carbon dioxide (PetCO.sub.2), after correcting for any deadspace, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveoli (PACO.sub.2) of the patient or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (PaCO.sub.2). A re-breathing process is typically employed either to non-invasively estimate the carbon dioxide content of mixed venous blood (in total re-breathing) or to obviate the need to know the carbon dioxide content of the mixed venous blood (by partial re-breathing). Re-breathing processes typically include the inhalation of a gas mixture which includes carbon dioxide. During re-breathing, the CO.sub.2 production of the patient is less than during normal breathing. Re-breathing during which the CO.sub.2 production decreases to near zero is typically referred to as total re-breathing. Re-breathing that causes some decrease, but not a total cessation of CO.sub.2 elimination, is typically referred to as partial re-breathing.
Re-breathing is typically conducted with a re-breathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide. FIG. 1 schematically illustrates an exemplary re-breathing circuit 50 that includes a tubular airway 52 that communicates air flow to and from the lungs of a patient. Tubular airway 52 may be placed in communication with the trachea of the patient by known intubation processes, or by connection to a breathing mask positioned over the nose and/or mouth of the patient. A flow meter 72, which is typically referred to as a pneumotachometer, and a carbon dioxide sensor 74, which is typically referred to as a capnometer, are disposed between tubular airway 52 and a length of hose 60, and are exposed to any air that flows through re-breathing circuit 50. Both ends of another length of hose, which is referred to as deadspace 70, communicate with hose 60. The two ends of deadspace 70 are separated from one another by a two-way valve 68, which may be positioned to direct the flow of air through deadspace 70. Deadspace 70 may also include an expandable section 62. A Y-piece 58, disposed on hose 60 opposite flow meter 72 and carbon dioxide sensor 74, facilitates the connection of an inspiratory hose 54 and an expiratory hose 56 to re-breathing circuit 50 and the flow communication of the inspiratory hose 54 and expiratory hose 56 with hose 60. During inhalation, gas flows into inspiratory hose 54 from the atmosphere or a ventilator (not shown). During normal breathing, valve 68 is positioned to prevent inhaled and exhaled air from flowing through deadspace 70. During re-breathing, valve 68 is positioned to direct the flow of exhaled and inhaled gases through deadspace 70.
During total re-breathing, the partial pressure of end-tidal carbon dioxide (PetCO.sub.2 or etCO.sub.2) is typically assumed to be equal to the partial pressure of carbon dioxide in the venous blood (PvCO.sub.2) of the patient, as well as to the partial pressures of carbon dioxide in the alveolar blood (PACO.sub.2) and in the arterial blood (PaCO.sub.2). The partial pressure of carbon dioxide in blood may be converted to the content of carbon dioxide in blood by means of a carbon dioxide dissociation curve.
In partial re-breathing, measurements during normal breathing and subsequent re-breathing are substituted into the carbon dioxide Fick equation. This results in a system of two equations and two unknowns (carbon dioxide content in the mixed venous blood and cardiac output), which can be solved for Q.sub.1 without requiring knowledge of the carbon dioxide content in the mixed venous blood.
Alternative differential Fick methods of measuring pulmonary capillary blood flow or cardiac output have also been employed. Such differential Fick methods typically include a brief change of PetCO.sub.2 and VCO.sub.2 in response to a change in effective ventilation. This brief change can be accomplished by adjusting the respiratory rate, inspiratory and/or expiratory times, or tidal volume. A brief change in effective ventilation may also be effected by adding CO.sub.2, either directly or by re-breathing. An exemplary differential Fick method that has been employed, which is disclosed in Gedeon, A. et al. in 18 Med. & Biol. Eng. & Comput. 411-418 (1980), employs a period of increased ventilation followed immediately by a period of decreased ventilation.
An exemplary non-invasive breath-by-breath, or continuous, method of estimating cardiac output based on the carbon dioxide Fick equation is disclosed in Miller et al., A Simple Method for the Continuous Noninvasive Estimate of Cardiac Output Using the Maxima Breathing System. A Pilot Study, Anaesth. Intens. Care 1997; 25:23-28 ("Miller"). The methods of Miller are conducted on the so-called "Maxima" breathing system, which is a valveless, disposable, universal breathing system that selectively substantially eliminates alveolar gas. Since alveolar gas is eliminated, use of the Maxima breathing system facilitates a series of assumptions which lead to the following form of the carbon dioxide Fick equation: EQU Q.sub.t =VF.multidot.FECO.sub.2 /(CaCO.sub.2 -CvCO.sub.2),
where VF is the flow of fresh gas into the lungs of the patient and FECO.sub.2 is the volumetric fraction of carbon dioxide in the expiratory tidal volume of the patient's respiration. In accordance with the method of Miller, VF is adjusted to achieve a substantially constant FECO.sub.2 value in the range of 4.0-4.2%. Thus, in order for this method to provide an accurate breath-by-breath estimate of cardiac output, the breathing or ventilation of a patient should be stable. Since the mean CaCO.sub.2 -CvCO.sub.2 in patients with normal hemoglobin concentrations is typically assumed to be approximately 4 volumes percent, FECO.sub.2 /(CaCO.sub.2 -CVCO.sub.2) is typically about one. Thus, the cardiac output of a patient may be determined in accordance with the method of Miller by determining the flow of fresh gas into the lungs of the patient. Because the method of Miller requires control over the fresh gas flow into the lungs of a patient:, it may not be employed to determine the cardiac output of non-ventilated patients or during or after changes in breathing. Moreover, the method of Miller requires that a particular type of ventilation apparatus be employed to measure the cardiac output of a patient.
Alternative methods disclosed in Miller that may be employed to measure the cardiac output of a patient also require a measurement of the concentration of hemoglobin in the blood of the patient and/or estimation of CO.sub.2 diffused in the plasma (i.e., not carried by hemoglobin). These alternative methods are somewhat undesirable in that measurement of hemoglobin concentration is typically invasive.
Thus, there is a need for a non-invasive method of continuously determining the cardiac output or pulmonary capillary blood flow of a patient that may be conducted during both normal, stable breathing and during or following changes in breathing or ventilation. There is also a need for a method of non-invasively, continuously determining the cardiac output or pulmonary capillary blood flow of both ventilated and non-ventilated patients.